Why for free?

Sometimes when you write a book, it makes a lot of sense to think big and act commercially. Thinking big in this sense means you expect many people to have access to your book. Acting commercially means that you try to successfully market and sell your book. Sometimes, it's enough to just think big, knowing that acting commercially will certainly keep everything small. When you write a book containing mathematical symbols, thinking big and acting commercially doesn't seem the right combination. I merely hope to see the material to be used by many students and instructors everywhere and to receive a lot of constructive feedback that will lead to improvements. Acting commercially has never been one of my strong points anyway.

However, freely accessible doesn't mean that everyone has the right to copy and spread the material, which I would find quite offensive. For this reason, when requesting an electronic copy, the book will be watermarked with your e-mail address. The watermark is part of the LaTeX source, so it will take some effort to remove, although I do not have the illusion that removal is impossible.